1. Field of the Invention
This invention relates to a quantizing circuit, and more particularly is applicable to a film scanner adapted to read image data from a negative or positive photographic film or the like.
2. Description of the Related Art
In an example of a conventional image data processing apparatus, a film scanner is used to read image data from a negative or positive photographic film (hereinafter referred to merely as "a negative or positive film", when applicable), and the image data thus read are processed as required.
That is, in a film scanner of this type, a one-dimensional image sensor is employed to pick up the image from a negative or positive film, and the resultant image pickup signal is converted into image data.
In this operation, the film changes in tone and in gamma characteristic as a whole depending on the image pickup conditions and the developing conditions. Therefore, in the image scanner, the image pickup signal is gamma-corrected using a non-linear amplifier circuit, and then is converted into image data.
In the case of a negative film, the gamma characteristic is on the order of 0.6. Therefore, as shown in FIG. 1, the contrast is expressed with a relatively small variation.
According to one method of representing a gradation of an image, .gamma. is expressed as a tangent of an angle between an axis representing an amount of exposure and the slope of a characteristic curve as shown in FIG. 1, and can be represented as follows: EQU G(.gamma.)=tan .theta. (1)
That is, if, in photographing a film, the exposure value is optimum, then the brightness (or density) of the film appears relatively low; whereas, if there is over-exposure, then the brightness of the film as a whole is relatively high, so that the image on the film appears to be "floating".
On the other hand, in forming a print from the film, a large difference in density cannot be properly expressed. Therefore, the density of the whole film is adjusted so that the dark and bright portions selected or desired by the camera operator appear on the print without change and that the resultant image is natural. Thus, in forming a print from a negative film, part of a wide range of densities is selectively used.
On the other hand, in the case of a film scanner, in order to adjust the gamma of the image pickup signal using a non-linear amplifier circuit, the operator performs a data setting operation; that is, he determines the part of the large range of densities which is to be selected, and selects a corresponding gamma value.
As a result, with an image processing apparatus of this type, even when a processed image is printed, the resultant print is natural in picture quality.
The above-described data setting operation is rather inconvenient, because the operator must perform the operation while watching the film.